It is convenient to describe the present invention with reference to an energy converter arranged to convert electrical energy from renewable energy sources to energy which is of a quality such that it may be delivered to either a low or medium voltage distribution network, the national grid, or the like. However the invention need not be so limited, and may apply to an energy converter used to convert electrical energy from any number of further energy sources.
With the continuing drive towards reducing CO2 emissions emitted in the atmosphere as a by-product of energy production facilities, there has been a recent drive towards the use of renewable energy sources. Such sources are generally emission free, however their power output can often be dependent upon external environmental factors, such as the level of sunlight when using a photovoltaic cell, wind strength when using a wind turbine, or the like. It can be advantageous therefore to have the flexibility to use any type of renewable source, and configuration of it, to maximise the potential energy yield from each site where such sources may be located.
Such flexibility is traded-off against having to undertake a complete and time-consuming re-design of the electrical system for each installation, including the choice of electrical energy converter, which would typically be designed to interface with a single renewable energy technology.
It is also often the case that the most appropriate configuration of a given technology is dictated by the surrounding structures and buildings, space availability and visual appearance. For example, a solar photovoltaic installation may operate either as a single solar PV cell array on a flat roof with no shadowing from adjacent structures, or may operate as multiple solar PV cell arrays on different aspects of a building with a pitched roof or where partial shading occurs. The former uses a single input to an energy converter, whilst the latter uses multiple independent inputs in order to capture peak levels of solar energy at different times of the day, as the sun's position changes. The electrical power from each input or source is transferred to the local network, or mains supply.
A common topology for an energy converter that allows bi-directional power flow between two networks of differing voltage frequency or magnitude is a back-to-back pair of Voltage Source Inverters (VSIs). Such energy converters are typically used to allow power flow between an electro-mechanical device and an electrical power distribution system. Whilst a back-to-back VSI converter is suitable for use with wind and hydro turbines, and other rotating AC power sources, the three-phase version, which is typically employed for powers above 5-10 kW, is not suitable for use on DC sources such as solar photovoltaic cell arrays.
Similar energy converters have previously been suggested to convert the varying power supplied by renewable sources. US2004/0125618 describes such an electrical energy converter however it does not address many of the issues detailed above.